Six important Types of Electromagnetic Radiations

Electromagnetic radiations range from 0.02 A in the case of gamma rays to several meters in the case of radio-waves. Based primarily on wave lengths and frequency electromagnetic radiations have been classified into a number of regions:

1. Grays:

Gamma rays are radiations of shortest wavelength (0.02-10 A), which are emitted by atomic nuclei. They have very high energy content. This region of electromagnetic spectrum finds little use spectroanalytical technology.

2. X-rays:

X-rays with wave length ranging from 1 A to 10 A are emitted or absorbed by movement of electrones close to the nuclei of relatively heavy atoms. X-ray spectroscopy, the branch of analytical chemistry which deals with this portion of electromagnetic radiations is very useful for quantitative analyses, qualitative analyses and structural determinations. The double helical structure of DNA molecule could be deduced only after X-ray defraction studies.

3. Ultra-violet radiations:

Ultra-violet rays have wavelengths ranging between 10 A to 4000 A. These are high energy radiations. Ultra-violet spectroscopy is the branch of science which deals with this zone of electromagnetic radiations and is very useful in quantitative and qualitative analyses as well as structural determinations of those compounds which absorb or emit radiations in the ultraviolet region of the spectrum (for example nucleic acids).

4. Visible light:

The visible portion of electromagnetic radiations, the portion which we can see with our naked eyes, extends from 4000 A to 7500 A. The perception of colour of an object by human eye is due to the fact that certain wave lengths of electromagnetic spectrum within the visible range are absorbed more effectively than others.

Things appear white because all wave lengths are reflected equally by the object. It is black because very little light of any wave length is reflected. It is blue because all other waves of light in the visible region are absorbed except that of blue region. The approximate wave-lengths corresponding to different colours are given. The branch of science dealing with this region of electromagnetic spectrum is known as visible spectroscopy or colorimetry or Spectrophotometry. It is one of the oldest physical methods used in analytical chemistry.

Approximate wave lengths corresponding to different colours of light

Colour Wavelengths (A)

Violet 4000-4350

Blue 4350-4800

Green-blue 4800-4900

Blue-green 4900-5000

Green 5000-5600

Yellow-green 5600-5800

Yellow 5800-5950

Orange 5950-6100

Red 6100-7500

5. Infra-red region:

This region of electromagnetic spectrum has wave lengths extending from 0.75 mm to 200 mm and is usually association with changes in the vibration of molecules. Infra-red spectroscopy, the branch of science which deals with this portion of electromagnetic spectrum is a very efficient tool of analytical chemistry as in addition to structural determinations it offers useful methods of qualitative as well quantitative analyses.

6. Micro-wave region:

Microwave region extends from 0.1 mm to 1 cm. This region of electromagnetic radiations is associated with rotation of molecules. Micro-wave spectroscopy which deals with this part of the spectrum is an asset in the study of molecules having a permanent dipole movement.